Process for producing a strained layer based on germanium-tin

ABSTRACT

The invention pertains to a process for producing a strained layer based on germanium-tin (GeSn). The process includes a step of producing a semiconductor stack containing a layer based on GeSn and having an initial strain value that is non-zero; a step of structuring the semiconductor stack so as to form a structured portion and a peripheral portion, the structured portion including a central section linked to the peripheral portion by at least two lateral sections having an average width greater than an average width of the central section; and a step of suspending the structured portion, the central section then having a final strain value higher than the initial value.

TECHNICAL FIELD

The field of the invention is that of the production of a tensionallystrained layer based on germanium-tin GeSn, with the aim notably ofobtaining a direct electronic band structure. The invention applies inparticular to the production of a microelectronic or optoelectronicdevice comprising a tensionally strained layer of this kind based onGeSn.

PRIOR ART

In various microelectronic or optoelectronic applications, it may beadvantageous to use a tensionally strained layer produced on the basisof germanium-tin. This is notably the case with high-performancetransistors, where the strain that the material undergoes is reflectedin an increase in the speed of displacement of the charge carriers,which improves the performance of a transistor of this kind. It is alsothe case with light sources such as electrically pumped lasers, forwhich the emitting layer based on germanium-tin may have a directelectronic band structure by applying a sufficient strain value.

The article by Wirths et al. entitled Tensely strained GeSn alloy asoptical gain media, Appl. Phys. Lett. 103, 192110 (2013) describes anexample of a process for producing a tensionally strained layer based ongermanium-tin. This process comprises growth of an intermediate thicklayer of Ge_(1-y)Sn_(y), partially or fully relaxed, on a germaniumnucleation layer, then growth of a so-called thin layer ofGe_(1-x)Sn_(x) of interest on the intermediate layer, the atomicproportion of tin x_(Sn) being less than that y_(Sn) of the intermediatelayer. The intermediate layer is said to be thick, in the sense that itsthickness is greater than the critical thickness, starting from whichthe mechanical stresses to which the layer is subjected relaxplastically. The intermediate thick layer of Ge_(1-y)Sn_(y) has, at thelevel of its upper surface, a lattice parameter greater than that of thelayer of Ge_(1-x)Sn_(x) of interest, which makes it possible to strainthe layer of interest. In this configuration, it is expected that a thinlayer of Ge_(1-x)Sn_(x) will have a direct electronic band structurewhen x_(Sn) is less than 10% and when y_(Sn) is greater than or equal to12%. The authors effectively obtain a thin layer with a directelectronic band structure for x_(Sn)=8% and y_(Sn)=12%.

However, this production process requires the fabrication of anintermediate layer with a high atomic proportion of tin for strainingthe Ge_(1-x)Sn_(x) layer of interest. Now, said layer with a high atomicproportion of tin and with good crystal quality is particularlydifficult to produce, on the one hand because of the large differencebetween the lattice parameter of germanium (a_(Ge)=5.658 Å) and that oftin (a_(Sn)=6.489 Å), and on the other hand because of the differencebetween the melting point of germanium (about 950° C.) and that of tin(about 240° C.), which may lead to demixing of the tin.

DISCLOSURE OF THE INVENTION

The invention aims to remedy the drawbacks of the prior art, at leastpartly, and more particularly proposes a process for producing aso-called tensionally strained layer of interest, based ongermanium-tin, not requiring production of an intermediate layer basedon germanium-tin with a high proportion of tin. For this purpose, theinvention relates to a process for producing a tensionally strainedlayer based on germanium-tin, comprising the following steps:

-   -   producing a semiconductor stack resting on a supporting layer        via a sacrificial layer, said semiconductor stack comprising a        nucleation layer and a so-called layer of interest based on        germanium-tin grown epitaxially starting from the nucleation        layer, said stack having a non-zero initial strain value;    -   structuring said semiconductor stack so as to form:        -   a structured part and a peripheral part, the structured part            comprising a central portion joined to the peripheral part            by at least two lateral portions opposite one another with            respect to the central portion,        -   the lateral portions having an average width greater than an            average width of the central portion;    -   suspending the structured part by etching the sacrificial layer        located beneath the structured part, the so-called suspended        central portion then having a final strain value greater than        the initial value.

Certain preferred but non-limiting aspects of this process are asfollows.

The process may comprise the following steps:

-   -   prior to the production step a), estimating a value of atomic        proportion of tin and of a first minimum strain value for which        the layer of interest has a direct electronic band structure;        and    -   determining a semiconductor stack comprising a nucleation layer        and said estimated layer of interest, and having a second        minimum strain value;    -   producing said semiconductor stack in such a way that it has        said non-zero initial strain value and so that the layer of        interest has an initial value lower than said first minimum        value;    -   determining the structuring in such a way that, after the step        of suspension, the central portion of the structured part has a        final strain value greater than or equal to said second minimum        value, said layer of interest then having a final strain value        greater than or equal to said first minimum value and then        having a direct electronic band structure.

The nucleation layer may be made of a semiconductor compound having aso-called natural lattice parameter lower than that of the materialbased on germanium-tin of the layer of interest.

The semiconductor stack may comprise at least one layer located betweenthe layer of interest and the nucleation layer made of a semiconductorcompound having a so-called natural lattice parameter less than or equalto that of the material based on germanium-tin of the layer of interest.

The semiconductor stack may have a thickness less than a so-calledcritical thickness.

Each layer of the semiconductor stack may have a thickness less than aso-called critical thickness.

The semiconductor stack may comprise upper and lower layers based ongermanium-tin, doped according to different types of conductivity,located on either side of the layer of interest, the latter not beingdoped intentionally.

Between the upper and lower doped layers on the one hand and the layerof interest on the other hand, there may be at least one so-calledbarrier layer based on germanium, or based on germanium-tin for whichthe atomic proportion of tin is lower than the value of the atomicproportion of tin in the layer of interest.

The process may comprise a step of partial etching of the nucleationlayer, this etching being selective with respect to the layer ofinterest, so as to remove the nucleation layer at the level of thecentral portion and preserve at least one part thereof at the level ofthe lateral portions.

The atomic proportion of tin in the layer of interest may be less than10%.

The process may further comprise a step of contacting the structuredpart with a surface freed from the supporting layer, so as to make thestructured part of the supporting layer integral by molecular bonding.

The process may further comprise the following steps:

-   -   determining a minimum value of energy of molecular bonding of        the structured part on the supporting layer, as well as a        minimum value of bonded surface area of the lateral portions,        these minimum values being such that said energy of molecular        bonding is greater than an elastic energy of the structured        part;    -   consolidation annealing at an annealing temperature such that        the energy of molecular bonding has a value greater than or        equal to said previously determined minimum value; and then    -   etching a so-called distal part of the lateral portions with        respect to the central portion, in such a way that the bonded        surface of the lateral portions has a value greater than or        equal to said previously determined minimum value.

The suspension step and the contacting step may be carried out byetching the sacrificial layer with HF in the vapor phase optionallyfollowed by deposition and then evaporation of a liquid between thesuspended structured part and the supporting layer, and in which, in theannealing step, the annealing temperature is greater than or equal to200° C.

The process may comprise, in the suspension step, oxidation or nitridingof a freed surface of the supporting layer as well as of a lower surfaceof the structured part oriented towards the free surface, and in which,in the annealing step, the annealing temperature is greater than orequal to 100° C.

Following the suspension step, dielectric layers, resulting from theoxidation or nitriding carried out, may be formed at the level of thestructured part and of the supporting layer, which have a thicknesspreferably greater than or equal to 10 nm.

The invention also relates to a process for producing a microelectronicor optoelectronic device comprising said layer of interest based ongermanium-tin obtained by the process according to any one of thepreceding features, in which a PN junction is produced in the layer ofinterest, or a p-i-n junction at the level of said layer of interest,the latter then not being doped intentionally.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, aims, advantages and features of the invention willbecome clearer on reading the following detailed description ofpreferred embodiments of the invention, given as non-limiting examples,and referring to the appended drawings, where:

FIGS. 1A, 1B and 1C are schematic views, in section (FIG. 1A and 1C) andfrom above (FIG. 1B), of a stack comprising a layer of interest based ongermanium-tin, for different steps of a process according to a firstembodiment;

FIGS. 2A, 2B, 2C are schematic views, in section (FIG. 2A and 2B) andfrom above (FIG. 2C), of a variant of the stack illustrated in FIGS. 1Ato 1C;

FIG. 3 illustrates a flowchart of a process according to a secondembodiment, for obtaining a layer of interest with a direct electronicband structure;

FIGS. 4A, 4B and 4C are schematic sectional views of a semiconductorstructure for different steps of a process according to a thirdembodiment;

FIGS. 5A and 5B are schematic top views of a semiconductor structure,with and without a peripheral part, respectively; and FIG. 5C is anexample of the relation between the bonding surface energy as a functionof the annealing temperature, for hydrophilic bonding and forhydrophobic bonding;

FIGS. 6A, 6B and 6C are schematic sectional views of a semiconductorstructure, for different steps of a process according to a fourthembodiment involving a step of hydrophilic bonding;

FIGS. 7A and 7B are schematic sectional views of an optoelectronicdevice for emitting incoherent light comprising a semiconductorstructure obtained by the process according to the fourth embodiment;

FIGS. 8A and 8B are schematic sectional views of an optoelectronicdevice for emitting coherent light comprising a semiconductor structureobtained by the process according to the fourth embodiment;

FIGS. 9A to 9F represent, schematically and in section, different stepsof an example of a process for producing a laser source comprising asemiconductor stack obtained by the process according to the first orthe second embodiment.

DETAILED ACCOUNT OF PARTICULAR EMBODIMENTS

In the figures and in the rest of the description, the same referencesrepresent elements that are identical or similar. Moreover, thedifferent elements are not shown to scale, for the sake of clarity ofthe figures. Moreover, the various embodiments and variants are notexclusive of one another and may be combined with one another. Unlessstated otherwise, the terms “approximately”, “about”, “of the order of”signify to within 10%.

The invention relates in general to a process for producing a so-calledlayer of interest based on tensionally strained germanium-tin, notablywith the aim of obtaining a direct electronic band structure.

“Strained layer” means a layer made of a single-crystal semiconductormaterial having mechanical strain in tension or in compression, leadingto distortion of the unit cells of the crystal lattice of the material.The layer is tensionally strained when it is subjected to a mechanicalstrain that tends to stretch the cells of the lattice in a plane. Thisis then reflected in the presence of a compressive strain oriented alongan axis approximately orthogonal to the plane of stretching. In thecontext of the invention, the layer of interest based on germanium-tinis intended to be tensionally strained in the plane of the layer, whichis reflected in the fact that its lattice parameter has a so-calledeffective value greater than its so-called natural value when thematerial is relaxed, i.e. not strained. In the rest of the description,unless stated otherwise, the strain in question is oriented in the planeof the layer.

“Layer of interest based on germanium-tin GeSn” means that the layer ofinterest is made of an alloy Ge_(1-x)Sn_(x) comprising germanium andtin. The germanium-tin alloy may be a binary alloy Ge_(1-x)Sn_(x),ternary, for example Si_(y)Ge_(1-x-y)Sn_(x), or even quaternary or more.The atomic proportion of tin in the alloy is designated X_(Sn). Thelayer of interest based on germanium-tin is preferably formed of analloy that is homogeneous in terms of the atomic proportions of theelements forming the alloy and in terms of the values of any optionaldoping.

“Direct or approximately direct electronic band structure” means thatthe minimum energy of the valley L (or indirect valley) is greater thanor approximately equal to the minimum energy of the valley Γ (or directvalley) of the conduction band, in other words:ΔE=E_(min,L)−E_(min,Γ)≥0. Here, “approximately equal” means that thisenergy difference is of the order of magnitude of or less than kT, wherek is the Boltzmann constant and T is the temperature of the material. Inthe context of the invention, initially the layer of interest isproduced on the basis of germanium-tin, the energy band structure ofwhich is indirect when the material is not tensionally strainedsufficiently, in other words ΔE<0, and may then have tensile strainsufficient to make its band structure direct.

FIGS. 1A, 1B and 1C illustrate different steps of a process forproducing the layer of interest based on tensionally strainedgermanium-tin, according to a first embodiment.

Here, and for the rest of the description, a direct three-dimensionalcoordinate system is defined (X,Y,Z), where the X and Y axes form aplane parallel to the plane of a supporting layer, and where the Z axisis oriented along the thickness of the layers. In the rest of thedescription, the terms “vertical” and “vertically” are understood asrelating to an orientation approximately parallel to the Z axis.Moreover, the terms “lower” and “upper” are understood as relating to anincreasing position as we move away from the supporting layer in the +Zdirection.

Referring to FIG. 1A, a semiconductor stack 10 is produced comprisingsemiconductor layers, including at least one nucleation layer 11 and thelayer of interest 12 based on germanium-tin, here made of germanium-tinbinary alloy Ge_(1-x)Sn_(x), the latter being grown epitaxially startingfrom the nucleation layer 11. The semiconductor stack 10 covers asacrificial layer 2 resting on a supporting layer 1.

The supporting layer 1 may be made of a semiconducting, electricallyconducting or dielectric material. This material may have a thickness ofthe order of some tens of nanometers to some hundreds of microns, forexample may be between 10 nm and 700 μm, or even between 500 nm and 100μm. It is made of silicon here, but it may be selected more generallyfrom, among others, silicon, sapphire, borosilicate, silica, glass,quartz.

The sacrificial layer 2 may be made of a material that is etchableselectively relative to the material of the supporting layer 1 and thematerials of the semiconductor stack 10. It may be a silicon oxide (forexample SiO₂) or a silicon nitride (for example Si₃N₄). The sacrificiallayer 2 may have a thickness of the order of some tens of nanometers tosome microns, for example between 10 nm and 10 μm, or even between 500nm and 5 μm. It is made of silicon dioxide SiO₂ here.

The nucleation layer 11 may be made of a single-crystal semiconductormaterial suitable for nucleation, or germination, of the layer ofinterest 12 based on GeSn. The material of the nucleation layer 11 maybe selected from the elements or compounds of column IV of the periodictable, such as germanium, silicon, tin, and the alloys formed from theseelements such as GeSn, SiGeSn, SiGe. It may also be selected from thecompounds comprising elements of columns III and V, such as GaP, AIP,AlAs, INGaAs, InP, AlGas, or even from the compounds comprising elementsof columns II and VI, such as ZnS, ZnSe, CdZnTe, CdTe.

The layer of interest 12 is made of a single-crystal semiconductormaterial based on germanium-tin, and is made of a germanium-tin binaryalloy Ge_(1-x)Sn_(x) here. The atomic proportion of tin X_(Sn) isnon-zero and may be between 1% and 14%, preferably between 4% and 10%.It has a thickness of the order of some tens of nanometres to somehundreds of nanometres or even to some microns, for example between 10nm and 1 μm.

Preferably, each of the layers forming the semiconductor stack 10 has athickness less than its so-called critical thickness starting from whichthe stresses to which the layer is subjected may relax and causestructural defects to develop, for example lattice mismatchdislocations, then leading to potential degradation of the electronicand/or optical properties of the layer. Also preferably, the thicknessof the semiconductor stack 10 is less than its critical thickness. Thisminimizes the degradation of the crystal quality of the semiconductorstack 10 and of the layers from which it is formed.

According to one embodiment, the nucleation layer 11 may be made of amaterial having a lattice parameter less than that of the layer ofinterest 12. This is notably the case with germanium Ge, and with agermanium-tin alloy with an atomic proportion of tin less than that ofthe layer of interest 12. Thus, the layer of interest 12 may have acompressive strain with respect to the nucleation layer 11, notably whenit is in contact with the nucleation layer 11 or when an intermediatelayer of the same material as the layer of interest 12 is locatedbetween the latter and the nucleation layer 11. The nucleation layer 11preferably has a thickness greater than that of the layer of interest12.

According to another embodiment, the nucleation layer 11 may be made ofa material having a lattice parameter greater than that of the layer ofinterest 12. This is notably the case with tin and with a germanium-tinalloy with an atomic proportion of tin greater than that of the layer ofinterest 12. Thus, the layer of interest 12 may be tensionally strainedwith respect to the nucleation layer 11. The nucleation layer 11preferably has a thickness less than that of the layer of interest 12.

In the context of the invention, the semiconductor stack 10 is producedin such a way that it has a non-zero tensile strain, i.e. the strain ofthe stack, in the (X,Y) plane, has a non-zero, positive initial valueσ_(s) ^(i). The strain of the stack 10 corresponds to the mean value ofthe strains of each layer of the stack, according to the relation:

σ_(s)=Σ_(k=1) ^(N) E _(k)ε_(k) e _(k)

where E_(k) is the Young's modulus of the layer k belonging to thestack, of thickness e_(k), and ε_(k) is the value of the deformationthat the layer k undergoes.

For this, the materials of the layers and their thicknesses are selectedin such a way that the strain of the stack 10 in the (X,Y) plane isstrictly positive, in other words in such a way that the stress σ_(s,z)^(i) along the Z axis is either in compression, and therefore negative,according to the following relation (1):

σ_(s,z) ^(i)=Σ_(k=1) ^(N) E _(k)ε_(k,z) ^(i) e _(k)<0   (1)

where ε_(k,z) ^(i) is the initial value of the deformation along the Zaxis that the layer k undergoes. This initial value ε_(k,z) ^(i) of thedeformation may be estimated conventionally from the lattice parametera_(o,k) of the relaxed layer k and the lattice parameter a_(z,k) of thestrained layer k from the relation ε_(k,z)^(i)=(a_(z,k)−a_(o,k))/a_(o,k). It may also be estimated conventionallyfrom the stiffness constants of the layer k and the deformation in the(X,Y) plane that the layer k undergoes.

As an example, the nucleation layer 11 may be a layer of germaniumdeposited or transferred onto a sacrificial layer 2 of silicon oxideresting on a supporting layer 1 of silicon. This assembly of layers ispreferably produced by the process described in the work of Reboud etal. titled Structural and optical properties of 200 mmgermanium-on-insulator (GeOI) substrates for silicon photonicsapplications, Proc. SPIE 9367, Silicon Photonics X, 936714 (Feb. 27,2015), which notably employs the Smart Cut™ technology.

For this, firstly a layer of crystalline germanium is grown epitaxiallyon a thick layer of silicon. The layer of germanium then has, at roomtemperature, a residual tensile strain of the order of 0.2%. Then adielectric layer, for example a silicon oxide, is deposited on the freesurface of the layer of germanium, and then implantation of H⁺ ions inthe layer of germanium is carried out. Next, the dielectric layercovering the layer of germanium is made integral with a handle substrateformed from a dielectric layer covering a layer of silicon. The layer ofgermanium is separated into two parts at the level of a zone weakened byion implantation. This results in a layer of single-crystal germanium 11covering a sacrificial layer 2, in this case of silicon oxide, whichrests on a supporting layer 1, for example a layer of silicon of a SOIsubstrate. This process is advantageous in that the crystal quality ofthe nucleation layer 11 is particularly high and approximatelyhomogeneous through the thickness of the layer. As an example, thedislocation density may be below 10⁷ cm⁻² over the entire thickness ofthe layer, in particular at the interface with the sacrificial layer 2.

A tensionally strained germanium nucleation layer 11 is thus obtained.Owing to the difference in values between the coefficients of thermalexpansion of germanium and silicon, after cooling to room temperature,the nucleation layer undergoes tensile strain in the (X,Y) plane of theorder of 0.2%, which is reflected in an effective lattice parameter ofabout 5.670 Å whereas the natural lattice parameter of relaxed germaniumis 5.658 Å.

Alternatively, the tensionally strained nucleation layer 11 may beproduced by epitaxial growth of a layer of germanium on a substrate, thelayer of germanium then being covered with a layer of silicon oxide.This stack is made integral by molecular bonding with a second stackformed from a layer of silicon covered with a layer of silicon oxide,bonding being effected by bringing the layers of silicon oxides intocontact with one another. Then the substrate is removed completely, forexample by grinding, thus obtaining the layer of germanium bound to asupporting layer 1 of silicon by a sacrificial layer 2 of silicon oxide.This approach is notably described in the work of Jan et al'. titledTensile-strained germanium-on-insulator substrate for silicon-compatibleoptoelectronics, Opt. Mater. Express 1, 1121-1126 (2011).

The layer of interest 12 based on GeSn is then grown epitaxiallystarting from the exposed surface of the nucleation layer 11, forexample by a technique of chemical vapor deposition (CVD), optionally atlow pressure (low-pressure chemical vapor deposition, LPCVD) or else bymolecular beam epitaxy (MBE).

The ratios of the flows of the precursor gases, for example Ge₂H₆ andSnCl₄, are controlled to obtain the value x_(Sn) of atomic proportion oftin in the layer of interest 12. As an illustration, the temperature ofgrowth may be between 300° C. and 400° C. and the rate of growth may beof the order of 10 nm/min to 100 nm/min. The layer of interest 12 thenhas a compressive strain in the (X,Y) plane as it has a natural latticeparameter greater than the effective lattice parameter of the nucleationlayer 11.

The thicknesses of the nucleation layer 11 and of the layer of interest12 are selected using relation (1) so that the semiconductor stack 10has a tensile stress σ_(s) ^(i)>0 in the (X,Y) plane, in other words acompressive stress σ_(s,z) ^(i)>0 along the Z axis.

As an illustration, we may thus obtain a semiconductor stack 10tensionally strained in the (X,Y) plane, and formed of:

-   -   the nucleation layer 11 of germanium with a thickness of for        example 1 μm, having a tensile strain of +0.2% (effective        lattice parameter of 5.670 Å against a natural lattice parameter        of 5.658 Å), and    -   the layer of interest 12 of Ge_(1-x)Sn_(x) with an atomic        proportion of tin x_(Sn) of 8%, with a thickness of for example        50 nm. The layer of interest 12 thus has an effective lattice        parameter of 5.670 Å, equal to that of the nucleation layer 11,        against a natural lattice parameter of 5.724 Å. It then has a        compressive strain in the (X,Y) plane of −0.94%.

Referring to FIG. 1B, structuring of the semiconductor stack 10 iscarried out so as to form a structured part 20 and a peripheral part 30,the structured part 20 comprising a central portion 21 joined to theperipheral part 30 by at least two lateral portions 22 opposite oneanother with respect to the central portion 21. Here, the structuredpart 20 comprises a single pair of tensioning arms 22 intended to ensurea subsequent increase in the uniaxial tensile stress of the centralportion 21, and therefore tensile stressing of the layer of interest 12located in the central portion 21. The structured part 20 is produced byconventional steps of optical and/or electronic lithography and thenetching of the stack, which therefore are not described in detail here.

The central portion 21 may have an approximately square or rectangularshape, in the (X,Y) plane, from some hundreds of nanometers to somemicrons in width, and from some hundreds of nanometers to some hundredsof microns in length. Other shapes are possible, such as polygonalshapes. The lateral portions 22, called tensioning arms 22 hereinafter,each connect an edge of the central portion 21 to the peripheral part30. They are arranged in pairs with respect to the central portion 21 soas to define at least one approximately rectilinear strain axis. Thus,an increase in tensile strain can be generated in the central portion 21in the subsequent step of suspension of the structured part 20, andtherefore tensile stressing of the layer of interest 12 located in thecentral portion 21.

For this, the tensioning arms 22 and the central portion 21 are formedso that the average width “b” of the tensioning arms 22 is greater thanthe average width “a” of the central portion 21, preferably ten timesgreater than the latter. “Width” means the transverse dimension of aportion or of an arm, in the (X,Y) plane, to its longitudinal axis.Here, the central portion 21 has an average width “a” oriented along theY axis and approximately constant along the longitudinal axis X. Thetensioning arms 22 have an average width “b” oriented here along the Yaxis.

Furthermore, the surface dimension in the (X,Y) plane is selected insuch a way that the tensioning arms 22 have little or no strain at theend of the subsequent step of suspension. More precisely, the localstrain decreases with increasing distance from the central portion 21and becomes negligible at a distance greater than or equal to one or twotimes a mean dimension of the central portion 21. The mean strain of thetensioning arms 22, i.e. the strain field integrated in the volume ofthe tensioning arms 22, has a value lower than that of the centralportion 21, or even is negligible with respect to the mean strain in thecentral portion 21. For the example in FIG. 1B, the tensioning arms 22have a trapezium shape so that the width increases with increasingdistance from the central portion 21. Other shapes are possible, forexample a shape where the tensioning arms 22 have an abrupt increase inwidth with respect to the central portion 21 and then a principal zonewith constant width.

The structuring may be carried out so as to control the value of theamplification of the tensile stress of the central portion 21 of thesemiconductor stack 10, obtained subsequently during suspension of thestructured part 20. For this, the dimensional parameters of thestructured part 20 are predetermined, for example the widths and lengthsof the central portion 21 and of the tensioning arms 22. As an example,in the case of a rectangular central portion 21, of length A andconstant width a, and rectangular tensioning arms 22 of length B/2-A/2and of constant width b, an amplification factor f linking the finaltensile stress σ_(s) ^(f) with the initial tensile stress σ_(s) ^(i) maybe expressed by relation (2) formulated in the article by Süess et al.titled Analysis of enhanced light emission from highly strainedgermanium microbridges, Nature Photon. 7, 466-472 (2013):

$\begin{matrix}{f = {\frac{{2L} + B}{B}{\left( {1 + \frac{A}{B - A}} \right)/\left( {\frac{a}{b} + \frac{A}{B - A}} \right)}}} & (2)\end{matrix}$

where L is the length of the sacrificial layer 2 removed under thestructured part 20 in the subsequent step of suspension. Thus, dependingon the dimensional parameters of the structured part 20 of thesemiconductor stack 10, it is possible to control the value of theamplification of the tensile stress applied to the central portion 21during suspension. The amplification factor may also be estimated usingnumerical simulation software such as COMSOL Multiphysics.

Referring to FIG. 1C (sectional view along line AA shown in FIG. 1B),suspension of the structured part 20 is carried out, which will causeamplification of the tensile stress on the central portion 21 of thesemiconductor stack 10, and therefore tensile stressing of the layer ofinterest 12 located in the central portion 21. For this, a cavity 3 ismade under the structured part 20 so as to suspend it above a freedsurface 4 of the supporting layer 1.

The cavity 3 is made by etching, for example by wet etching, of thesacrificial layer 2 made accessible by openings obtained during thestructuring of the semiconductor stack 10. Here, wet etching useshydrofluoric acid (also called HF, for hydrogen fluoride) in the vaporphase. The flow of HF vapor may be low so as to etch the sacrificiallayer 2 at a moderate rate of the order of 10 nm per minute. Moreprecisely, the flow of vapor may, as an example, comprise hydrofluoricacid at 15 torr of partial pressure, alcohol at 0.01 torr and nitrogenat 60 torr. Thus, the part of the sacrificial layer 2 located beneaththe structured part 20 is etched on its entire thickness. The structuredpart 20 is then suspended above the freed surface of the supportinglayer 1, thus forming a cavity 3. The cavity 3 is therefore locatedbetween the structured part 20 and the free surface 4 of the supportinglayer 1.

A suspended structured part 20 is thus obtained, whose tensioning arms22 keep the central portion 21 above the free surface 4 of thesupporting layer 1 and generate, in the central portion 21, an increasein initial tensile stress σ_(s) ^(i), along the strain axis or axes,owing to the difference in average width between the tensioning arms 22and the central portion 21. In this example, the strain of the centralportion 21 may be sufficient to obtain a direct electronic bandstructure of the layer of interest 12 located in the central portion 21.

The process thus makes it possible to obtain an increase in tensilestress of the central portion 21 of the semiconductor stack 10, andtherefore produce a tensile strain of the layer of interest 12 locatedin the central portion 21, without having to produce an intermediatelayer based on germanium-tin with a high proportion of tin, as describedin the article of Wirths et al. 2013 mentioned above. The crystalquality of the semiconductor stack 10, and in particular that of thelayer of interest 12 based on GeSn, may be preserved when the stack andits constituent layers have a thickness less than the respectivecritical thicknesses.

FIGS. 2A and 2B are schematic cross-sectional views of differentvariants of the stack illustrated in FIG. 1A, and FIG. 2C is a schematictop view of a variant of the structured part 20 illustrated in FIG. 1B.

In FIG. 2A, the stack 10 differs from that illustrated in FIG. 1A inthat it further comprises two layers 13A, 13B based on germanium-tinGeSn between which the layer of interest 12 is located in contact. Thelayers 13A, 13B based on germanium-tin may be made of a germanium-tinbinary alloy, or even ternary or quaternary, and may have an atomicproportion of tin identical to or different from that of the layer ofinterest 12. In this example, the layers 13, 13B are made of a materialidentical to that of the layer of interest 12, namely a germanium-tinbinary alloy with the same atomic proportion of tin, and differ from itby the doping. In fact, here the layer of interest 12 is notintentionally doped whereas the lower layer 13A has doping according toa first type of conductivity, for example of n-type, and the upper layer13B has doping according to a second type of conductivity opposite tothe first type, for example of p-type. The doped layers 13A, 13B mayhave different thicknesses, here lower than that of the layer ofinterest 12. As an example, the thickness of the layer of interest 12may be of about 350 nm and those of the doped layers 13A, 13B of about100 nm. The doped layers 13A, 13B may form, with the layer of interest12, a diode of the p-i-n type.

The stack 10 may further comprise an upper layer 14 intended to ensurebalancing of the stresses along the Z axis of the semiconductor stack10, with the aim of limiting the risks of overall deformation of thestack along the Z axis, of the buckling type. This upper layer 14 has athickness, a Young's modulus and a tensile stress in the (X,Y) planesuch that the vertical stress distribution along the Z axis isapproximately symmetric. More precisely, a vertical stress distributionis called approximately symmetric when the semiconductor stack 10 issuch that the sums Σ_(k)E_(k)ε_(k,Z)e_(k) relating to the layers locatedon either side of a mid-plane (parallel to the XY plane) of the stack 10are equal to one another to within 10%. Preferably, the upper balancinglayer 14 is made of the same material as the nucleation layer 11 and hasa product, thickness times tensile stress, approximately equal to thatof the nucleation layer 11. In this example, the nucleation layer 11 ismade of germanium with a thickness of about 1 μm and has a tensilestrain in the (X,Y) plane of about 0.2%. The upper balancing layer 14 ismade of germanium with a thickness of 800 nm and has a tensile strain ofabout 0.25%.

In FIG. 2B, the stack 10 differs from that illustrated in FIG. 2A inthat the layer of interest 12 is separated from the doped layers 13A,13B by two intermediate, so-called barrier layers 15A, 15B which have abandgap energy greater than that of the layer of interest 12, so as toimprove the quantum confinement of the charge carriers in the layer ofinterest 12. The barrier layers 15A, 15B may be made of a material thatis not intentionally doped, preferably of germanium or of agermanium-tin alloy, with an atomic proportion of tin less than that ofthe layer of interest 12. Of course, generally the semiconductor stack10 may have more semiconductor layers, based or not based ongermanium-tin, doped or not doped, and therefore may comprise severallayers of interest 12.

In FIG. 2C, the structured part 20 differs from that illustrated in FIG.1B in that two pairs of tensioning arms 22 of identical dimensions areshown, which makes it possible to generate a biaxial increase in thetensile strain of the central portion 21, in this case of approximatelyequal intensity along the two strain axes, here respectively parallel tothe X and Y axes. As a variant, each pair of tensioning arms 22 may havedifferent dimensions, so as to deform the central portion with adifferent intensity along each of the strain axes.

As a variant, the process may also comprise a step of partial etching ofthe nucleation layer 11, the etching being selective with respect to thelayer of interest 12, in order to obtain a central portion 21 no longercomprising the nucleation layer 11. Thus, the central portion 21 may beformed essentially of the layer of interest 12, and, if applicable,doped layers 13A, 13B and optionally barrier layers 15A, 15B. This stepof selective etching may be isotropic wet etching of the nucleationlayer 11, for example with carbon tetrafluoride (CF₄). The etching timeis adapted for etching the thickness of the nucleation layer 11, whichmay be of the order of half the width “a” of the central portion 21.Thus, at the end of this step of selective etching, the nucleation layer11 is no longer present in the central portion 21 whereas it is stillpresent at the level of the tensioning arms 22. The aforementioned upperbalancing layer 14 may then be omitted since the central portion 21 doesnot comprise the nucleation layer 11: the vertical stress distributionalong the Z axis is approximately symmetric.

FIG. 3 illustrates a flowchart of a process according to a secondembodiment, for obtaining a layer of interest 12 based on germanium-tinwith a direct electronic band structure.

In a first step 110, the minimum value σ_(ci) ^(f,min) of tensile stressfor obtaining a direct electronic band structure is estimated for alayer of interest 12 based on germanium-tin Ge_(1-x)Sn_(x) with anatomic proportion of tin x_(Sn), in other wordsΔE=E_(min,L)−E_(min,Γ)≥0. The minimum value σ_(ci) ^(f,min) of tensilestress may be estimated on the basis of the article by Gupta et al.titled Achieving direct band gap in germanium through integration of Snalloying and external strain, J. Appl. Phy., 113, 073707 (2013), whichillustrates an example of variation of ΔE as a function of the value ofthe tensile stress acting on the layer of interest 12 of Ge_(1-x)Sn_(x)and the atomic proportion of tin. This variation is determined on thebasis of a non-local empirical pseudopotential method (NL-EPM).

In a second step 120, the semiconductor stack 10 comprising such, alayer of interest 12 determined beforehand, with properties {x_(Sn);σ_(ci) ^(f,min)}, and having a tensile stress of value σ_(s) ^(f,min),is determined. The values σ_(s) ^(f,min) and σ_(ci) ^(f,min) in arepositive provided the stress in the (X,Y) plane is in tension.

For this, the transfer function for determining the value σ_(ci)^(f,min) of the layer of interest 12 from the value σ_(s) ^(f,min) ofthe semiconductor stack 10, in other words: σ_(ci) ^(f,min)=G(σ_(s)^(f,min)), is designated G. The transfer function G is parameterized bythe characteristics of different layers making up the semiconductorstack 10, namely the lattice parameters, the thicknesses of the layersand the respective Young modulus and Poisson modulus. Therefore theparameters of the transfer function G are determined, for example bynumerical simulation using the COMSOL Multiphysics software, or bysatisfying the following relation:

σ_(s,z) ^(f,min)=Σ_(k=1) ^(N) E _(k)ε_(k,z) ^(f,min) e _(k)

In a third step 130, said previously determined semiconductor stack 10is produced in such a way that:

-   -   the semiconductor stack 10 has an initial tensile stress as        σ_(s) ^(i)>0 lower than the value σ_(s) ^(f,min), and so that    -   it comprises the layer of interest 12 with atomic proportion of        tin x_(Sn) and with a value of initial stress σ_(ci) ^(i) lower        than σ_(ci) ^(min) in such a way that the band structure is        indirect, and the initial value σ_(ci) ^(i) may be positive (in        tension), zero or negative (in compression).

In a fourth step 140, structuring of the semiconductor stack 10 isdetermined so as to form the structured part 20 described above. Thestructuring is determined in such a way that suspension of thestructured part 20 causes an increase in stress of the central portion21, from the initial value σ_(s) ^(i) as to the final value σ_(s) ^(f),the latter then being greater than or equal to σ_(s) ^(f,min).

For this, the transfer function for passing from as σ_(s) ^(i) to σ_(s)^(f) is designated F, in other words: σ_(s) ^(f)=F(σ_(s) ^(i)). Thetransfer function F is parameterized essentially by the dimensions ofthe structured part 20, and notably by the average width of thetensioning arms 22 and of the central portion 21. The transfer functionmay be identical or similar to the amplification factor f mentionedabove with reference to relation (2). Therefore the parameters of thetransfer function F are determined, for example by numerical simulationusing the COMSOL Multiphysics software or by satisfying theaforementioned relation (2).

In a fifth step 150, the structured part 20 is suspended by etching thesacrificial layer 2 located beneath the structured part 20. Thus, at thesame time:

-   -   the central portion 21 of the stack passes from the initial        value σ_(s) ^(i) to the final value σ_(s) ^(f)=F(σ_(s) ^(i)) of        stress in the (X,Y) plane, the final value σ_(s) ^(f) then being        greater than or equal to σ_(s) ^(f,min);    -   the layer of interest 12 passes from the initial value σ_(ci)        ^(i) to a final value σ_(ci) ^(f)=G(σ_(s) ^(f)) of stress in the        (X,Y) plane, this final value then being greater than or equal        to σ_(ci) ^(f,min).

We thus obtain a layer of interest 12 based on germanium-tin, located atthe level of the central portion 21, which has a direct electronic bandstructure, without having had to produce a tensioning layer with a highatomic proportion of tin as described in the article of Wirths 2013cited above. It is then possible to obtain a direct electronic bandstructure for a layer of interest 12 based on GeSn having an atomicproportion of tin less than 14%, or even less than 10%, or even lessthan 8%, or even less. As before, the stack and the layers from which itis formed may have a crystal quality that is preserved when therespective thicknesses are less than the critical thicknesses.

FIGS. 4A to 4C illustrate different steps of a process for producing asemiconductor structure 40 according to a third embodiment, thesemiconductor structure 40 comprising the stack 10 with a layer ofinterest 12 based on germanium-tin described above, and being madeintegral by direct bonding on the supporting layer 1. This process issimilar to that described in application FR1559283 filed on 30 Sep.2015, which is incorporated here in its entirety by reference. Moreover,for clarity, only the semiconductor stack 10 is illustrated, but not thenucleation layer 11 and the layer of interest 12.

After the step of suspension described above, the central portion 21 ofthe semiconductor stack 10 is tensionally strained and then has elasticenergy Ee, which, to a first order, may be written: Ee˜σ_(s) ^(f)·ε_(s)^(f)·V, where σ_(s) ^(f) is the mean value of the tensile stress in the(X,Y) plane, ε_(s) ^(f) is the mean value of the strain corresponding tothe stress applied, and V is the volume of the central portion 21.

Direct bonding, also called molecular bonding or molecular adhesionbonding, means the integration of two surfaces of identical or differentmaterials against one other, without supplying an adhesive layer (suchas adhesive, glue, etc.), but by means of the attractive forces ofatomic or molecular interaction between the surfaces to be bonded, forexample van der Waals forces, hydrogen bonds, or even covalent bonds.The semiconductor stack 10, attached by direct bonding on the supportinglayer 1, then has a bonding energy which, to a first order, may bewritten: Ec˜E_(s)·S, where E_(s) is the bonding surface energy (it isassumed here that the surfaces to be bonded have an approximately equalsurface energy) and S is the extent of the bonded surfaces.

As is described in detail hereunder, the molecular bonding used here maybe of the hydrophilic or hydrophobic type. Bonding is of the hydrophilictype when it concerns the adherence of hydrophilic surfaces, i.e.surfaces having the capacity to bind to water molecules by an adsorptionmechanism. Bonding then involves hydrogen bonding forces, which have aparticularly high intensity of interaction. For this, the hydrophilicsurfaces are terminated with hydroxyl groups (—OH). Alternatively,bonding may be of the hydrophobic type and then concerns surfaces thatdo not have the capacity to adsorb water. For this, the hydrophobicsurfaces may be saturated with atoms such as hydrogen or fluorine.

For purposes of illustration, a process is described below for producinga semiconductor structure, comprising the stack with the layer ofinterest 12 based on germanium-tin located in the structured part 20,the semiconductor structure 40 being bonded by molecular adhesion to asupporting layer 1 of silicon.

According to a first step, a semiconductor stack 10 is producedcomprising the structured part 20 (not shown), resting on the supportinglayer 1 via a sacrificial layer 2.

The semiconductor stack 10 comprises the layer of interest 12 based ongermanium-tin (not shown), and is identical or similar to thesemiconductor stacks described above.

According to a second step illustrated in FIG. 4A, a cavity 3 is madeunderneath the structured part 20 so as to suspend it above a freedsurface of the supporting layer 1. The semiconductor stack 10 thenpasses from the initial value σ_(s) ^(i) to the final value σ_(s) ^(f)of stress in the (X,Y) plane, the final value then being greater than orequal to σ_(s) ^(f,min). The layer of interest 12 passes from theinitial value σ_(ci) ^(i) to a final value σ_(ci) ^(f) of stress in the(X,Y) plane, this final value preferably being greater than or equal toσ_(ci) ^(f,min), in such a way that the layer of interest 12 has adirect electronic band structure.

According to a third step illustrated in FIG. 4B, the structured part 20is brought into contact with the free surface 4 of the supporting layer1. For this, the structured part 20 may be immersed in a liquidsolution, for example of alcohol or of acidified deionized water (pHclose to 2), and then the liquid is evaporated. In the evaporation step,the structured part 20 comes into contact naturally with the freesurface 4 of the supporting layer 1. Thus, the structured part 20 restson the supporting layer 1, so that the lower surface of at least onepart of the tensioning arms 22 is in contact with the free surface 4 ofthe supporting layer 1. The lower surface of the central portion 21 maybe totally, partially, or not in contact with the free surface 4.

Bringing these surfaces into contact ensures direct bonding of thestructured part 20 with the supporting layer 1, of the hydrophobic typehere since the surfaces become attached to one another by means ofhydrogen bonds. At room temperature, as illustrated by FIG. 5C, showingthe variation of the surface energy of hydrophobic bonding between thebonded surfaces, the energy of hydrophobic bonding is in this case ofthe order of 5 mJ/m².

“Bringing into contact” means contact of the lower surface 23 of thestructured part 20 with the free surface 4 of the supporting layer 1.These surfaces may be formed of the material mainly making up the layersor of an interposed material different from this main material. Thestructured part 20 and the supporting layer 1 may thus comprise a thinlayer of an interposed material obtained for example by deposition or byoxidation, preferably after formation of the cavity 3. In the processdescribed here, involving hydrophobic bonding, the structured part 20and the supporting layer 1 do not comprise any interposed material.

Thus, a structured part 20 of the semiconductor stack 10 is obtained,bonded on the free surface 4 of the supporting layer 1. The bondedstructured part 20 comprises the central portion 21 and a part of thetensioning arms 22. The non-bonded part of the tensioning arms 22 islocated in the zone where the latter meets the peripheral part 30 of thesemiconductor stack 10, the latter resting on the unetched part of thesacrificial layer 2.

As a variant, the step of suspension and of bringing the structured part20 into contact with the free surface 4 of the supporting layer 1 may becarried out at the same time. For this, the cavity 3 is for example madeby wet etching with liquid HF or even with HF vapor at high pressure. Inthe case of etching with HF vapor, the flow of vapor may comprisehydrofluoric acid at 60 torr of partial pressure, alcohol at 0.1 torrand nitrogen at 75 torr. The flow of gas then leads to a higher etchingrate than that mentioned above, for example of the order of 100 nm/min,in a non-equilibrium etching reaction. Thus, drops of water and ofhydrofluoric acid, produced by the chemical reaction, form in the cavity3 and, on evaporating, cause the structured part 20 to come into contactwith the free surface 4 of the supporting layer 1.

At the end of this step, the bonded structured part 20, formed from thecentral portion 21 and the tensioning arms 22 resting on the supportinglayer 1, has:

-   -   a bonding energy Ec, resulting from hydrophobic molecular        bonding on the supporting layer 1. It may be estimated, to a        first order, from the relation:

E _(c) ≈E _(s)(S _(bt) +s _(pc))

-   -   where E_(s) is the surface energy evaluated from the relation        illustrated in FIG. 5C, and S_(bt) and S_(pc), are the        respective bonded surfaces of the tensioning arms 22 and of the        central portion 21. The bonding energy tends to stabilize the        bonded structured part 20 and prevent any stress relaxation        liable to alter its mechanical durability as well as its        crystalline structure, and hence degrade its electrical and/or        optical properties;    -   an elastic energy Ee, resulting from the tensile stress linked        to the strain of the central portion 21 by the tensioning arms        22. It may be estimated, to a first order, by the relation:

E _(e) ≈e(σ_(s,bt) ^(f)·ε_(s,bt) ^(f) ·S _(bt)+σ_(s,pc) ^(f)·ε_(s,pc)^(f) ·S _(pc))

-   -   where e is the mean thickness of the semiconductor stack 10,        σ_(s,bt) ^(f) and σ_(s,pc) ^(f) are the mean values of the        tensile stresses acting, respectively, on the tensioning arms 22        and the central portion 21, ε_(s,bt) ^(f) and ε_(s,pc) ^(f) are        the mean values of the corresponding strains. The elastic energy        tends to destabilize the bonded central part so as to relax the        stresses naturally.

It may be noted that, to a first order, the bonding energy comprises apredominant term linked to the bonded surface of the tensioning arms 22,the latter is generally greater than the bonded surface of the centralportion 21. Moreover, the elastic energy comprises a predominant termlinked to the strain of the central portion 21, insofar as thetensioning arms 22 have a mean value of strain close to the value ofresidual strain, the latter being lower than the value of the strain towhich the central portion 21 is subjected.

With the aim of producing a bonded semiconductor structure 40, whosemechanical durability and therefore the electrical and/or opticalproperties are preserved, which may be separated from the peripheralpart 30, the bonding energy must be greater than the elastic energy,which is reflected to first order in the following inequality:

E _(s) ^(min)(S _(bt) ^(min) +S _(pc))>e(σ_(s,bt) ^(f)·ε_(s,bt)^(f)·S_(bt) ^(min)+σ_(s,pc) ^(f)·ε_(s,pc) ^(f) ·S _(pc))

For this, both the minimum value of bonding surface energy E_(s) ^(min)and the minimum value of bonded surface S_(bt) ^(min) of the tensioningarms 22, necessary for satisfying this inequality, are determined. Ofcourse, this inequality may be stated using more detailed expressionsfor the bonding energy and elastic energy, for example by integratingthe stress field in the whole volume of the bonded structured part 20.

According to a fourth step, the molecular bonding of the structured part20 on the supporting layer 1 is reinforced, so as to obtain a value Esof bonding surface energy greater than or equal to the previouslydetermined minimum value E_(s) ^(min). For this, a thermal treatment iscarried out, in the form of consolidation annealing, in which the stackis subjected to an annealing temperature Tr for some minutes to somehours. As an illustration, the annealing temperature may be 200° C.applied for 2 h, which in this case increases the surface energy ofhydrophobic bonding from 5 mJ/m² to 100 mJ/m². The annealing temperatureis between a minimum value, which depends notably on the minimum bondedsurface S_(bt) ^(min) of tensioning arms 22 that we wish to conserve,and a maximum value that depends notably on the crystal quality of thesemiconductor stack 10 to be preserved. The maximum value of theannealing temperature may thus be below the temperature of epitaxialgrowth of the semiconductor stack 10. Thus, a structured part 20 isobtained, bonded to the supporting layer 1 with a bonding energy Ecwhose value is greater than or equal to the predetermined minimum value.We are then able to remove a part of the tensioning arms 22 to separatethe structured part 20 from the peripheral part 30.

In a fifth step illustrated in FIG. 4C, a distal portion 24 of thetensioning arms 22 is removed by etching, so as to separate, orindividualize, the structured part 20 relative to the peripheral part30. Here, “separate”, “make separate” or “individualize” means that thestructured part 20 is no longer joined to the peripheral part 30 by thetensioning arms 22. Moreover, distal portion 24 of the tensioning arms22 relative to the central portion 21 means the zone of the tensioningarms 22 remote from the central portion 21 and forming the connectionwith the peripheral part 30. The distal portion of the tensioning arms22 is removed by conventional operations of optical and/or electroniclithography and etching, so that the bonded structured part 20 comprisesa value of bonded surface S_(bt) of the tensioning arms 22 greater thanor equal to the previously determined minimum value S_(bt) ^(min). Thus,the bonded surface of the tensioning arms 22 is sufficient for thebonded structured part 20 to have a bonding energy greater than itselastic energy. Thus, a semiconductor structure 40 is obtained with acentral portion 21 bonded on the supporting layer 1 whose mechanicaldurability is assured, thus preserving its electrical and/or opticalproperties. The semiconductor structure 40 has a high crystal qualityand the central portion 21 has a predetermined mean strain. It is madeintegral with the supporting layer 1 by molecular bonding, whose bondingenergy and the bonded surface of the tensioning arms 22 make it possibleto fix the stress field. In the central portion 21, the layer ofinterest 12 based on GeSn has a direct electronic band structure.

FIG. 5A illustrates the semiconductor structure 40 with a centralportion 21, obtained by separating the structured part 20 from theperipheral part 30, by etching of the distal zone of the tensioning arms22 making the connection to the peripheral part 30. FIG. 5B illustratesthe semiconductor structure 40 obtained from the bonded structured part20 by etching the zone connecting the tensioning arms 22 to theperipheral part 30 (the latter also being removed).

FIG. 5C illustrates an example of the relation between the bondingsurface energy between a germanium surface of the nucleation layer 11and a silicon surface of the supporting layer 1, as a function of theannealing temperature, in the case of bonding of the hydrophilic typeand bonding of the hydrophobic type. Up to about 600° C., the bondingsurface energy has a lower value in the hydrophobic case than in thehydrophilic case. The tendency is then reversed starting from about 600°C. Moreover, in the hydrophilic case, the surface energy increases onceannealing at about 100° C. is applied, changing from energy of the orderof 100 mJ/m ² at room temperature to 1 J/m² after annealing at about200° C. In the hydrophobic case, it changes from energy of the order of5 mJ/m² at room temperature to 100 mJ/m² after annealing at about 200°C.

As a variant, according to a fourth embodiment described with referenceto FIGS. 6A to 6C, it is possible to perform molecular bonding of thehydrophilic type for the structured part 20 on the supporting layer 1,by a process identical or similar to that described in applicationFR1559283 filed on 30 Sep. 2015 cited above.

Referring to FIG. 6A, prior to contacting the structured part 20suspended on the supporting layer 1, a step of surface treatment of thestructured part 20 and of the free surface 4 of the supporting layer 1is carried out, with the aim of subsequently ensuring hydrophilicmolecular bonding of these elements. In this step, the surface 23 of thestructured part 20 opposite the cavity 3 and the free surface 4 of thesupporting layer 1 are treated, in such a way that they are each formedfrom a thin layer 41A, 41B of oxide or nitride, with a thickness fromsome nanometers to some tens of nanometers.

According to one variant, the structured part 20 and the supportinglayer 1 are covered, at the level of the cavity 3, with a so-calledinterposed thin layer 41A, 41B of oxide produced by oxidation, forexample obtained by exposing this zone of the stack to the open air fora sufficient time, for example 1 h. They may also be obtained by an O₃plasma oxidation technique, for example at room temperature, or even byan O₂ plasma oxidation technique, for example at a temperature 250° C.According to another variant, the thin layers of oxide or of nitride areobtained by a thin layer deposition technique, for example of the ALD(atomic layer deposition) type, whether or not plasma-assisted.

Referring to FIG. 6B, the structured part 20 is then brought intocontact with the free surface 4 of the supporting layer 1, for exampleby immersing the structured part 20 suspended in a liquid solution, forexample of alcohol or of acidified deionized water (pH close to 2), andthen evaporating the liquid. Bringing these surfaces into contactensures direct bonding of the hydrophilic type of the structured part 20on the supporting layer 1 at the level of the respective interposedlayers 41A, 41B. At room temperature, as illustrated in FIG. 5C, theenergy of hydrophilic bonding is in this case of the order of 100 mJ/m².These interposed layers 41A, 41B have a thickness of the order of sometens of nanometers to one or more hundreds of nanometers, and areadvantageously dielectric and can provide electrical insulation of thecentral portion 21 with respect to the supporting layer 1.

Next there is a step of determining the minimum value of bonding surfaceenergy E_(s) ^(min), here hydrophilic, and the minimum value of bondedsurface S_(bt) ^(min) of the tensioning arms 22, necessary for theenergy of hydrophilic bonding of the structured part 20 to be greaterthan the elastic energy of this same structured part 20.

This is then followed by a step of reinforcement of the molecularbonding of the structured part 20 attached to the supporting layer 1, soas to obtain a value Es of surface energy of hydrophilic bonding greaterthan or equal to the previously determined minimum value E_(s) ^(min).For this, a thermal treatment is carried out, in the form ofconsolidation annealing, in which the stack is subjected to an annealingtemperature Tr for some minutes to some hours. As an illustration, theannealing temperature may be 200° C. applied for 2 h, which in this caseincreases the surface energy of hydrophilic bonding from 100 mJ/m² to 1J/m².

Referring to FIG. 6C, a distal part 24 of the tensioning arms 22 isremoved by etching, so as to individualize the structured part 20 withrespect to the peripheral part 30. This step is similar to the stepdescribed above with reference to FIG. 5C and is not described in moredetail here. Thus, a semiconductor structure 40 is obtained with acentral portion 21 bonded by hydrophilic molecular adhesion on thesupporting layer 1, whose mechanical durability is ensured and theelectrical and/or optical properties are preserved. The middle layercomprises the layer of interest 12 based on GeSn, the band structure ofwhich is advantageously direct.

The process according to this embodiment therefore differs from theprocess described above essentially by the hydrophilic bonding, theintensity of which is greater than that of the hydrophobic bonding up toannealing temperatures of the order of 500° C. to 600° C., and by thepresence of an interposed layer 41A, 41B of an oxide or nitride at theinterface between the structured part 20 and the supporting layer 1,whose dielectric property provides electrical insulation between theseelements. This interposed material, besides a function of electricalinsulation, may also provide a function of removal of any heat producedat the level of the central portion 21, in the case when the latterforms an emitting layer of a light source.

Advantageously, a plurality of semiconductor structures 40 may beproduced collectively and simultaneously, starting from the samesemiconductor stack 10. The semiconductor structures are then adjacentand separated from one another. Thus, each semiconductor structure 40 isseparate from its neighbors, i.e. not attached to the correspondingperipheral part 30 of the same semiconductor stack 10.

The production of various optoelectronic devices comprising thesemiconductor structure 40 with a layer of interest 12 based ontensionally strained germanium-tin, obtained by one or other of theprocesses described above, will now be described.

FIGS. 7A and 7B show schematic sectional views of two examples of anoptoelectronic device for emitting incoherent light. The optoelectronicdevice is in this case a light-emitting diode.

In FIG. 7A, the light-emitting diode in this case comprises asemiconductor structure 40 obtained by the production process accordingto the fourth embodiment, i.e. involving hydrophilic molecular bonding.

The semiconductor structure 40 comprises a central portion 21 in tensionin which the layer of interest 12 based on GeSn (not shown) preferablyhas a direct electronic band structure. It is attached to the supportinglayer 1 by hydrophilic molecular bonding, which is reflected in thepresence of an interposed material 41A, 41B, here a silicon oxide,located at the interface between the germanium of the nucleation layer11 (not shown) of the semiconductor stack 10 and the silicon supportinglayer 1. The supporting layer 1 is in this case an upper silicon layerof a substrate, for example of the SOI type. It rests on a layer ofoxide 5 located between the supporting layer 1 and a thick lower layer 6of silicon.

The semiconductor structure 40 further comprises an encapsulation layer42 that covers the central portion 21 and the tensioning arms 22. Thisencapsulation layer 42 may be made of a dielectric material having goodthermal conductivity, such as Al₂O₃ or Si₃N₄. Si₃N₄ may, moreover, helpto induce a tensile stress in the semiconductor stack 10. The centralportion 21 comprises a p-i-n junction produced by implantation ofdopants (phosphorus and boron) so as to form an n-doped zone 45 close toa p-doped zone 43. Here, an intrinsic zone 44 (not intentionally doped)separates the n-doped and p-doped zones. The p-i-n junction extendsapproximately vertically across the central portion 21 and therefore thelayer of interest 12 based on GeSn (not shown), towards the supportinglayer 1. Moreover, two studs 46A, 46B of an electrically conductingmaterial are present at the level of the doped zones, forming electricalcontacts.

The light-emitting diode may be obtained in the following way. First,the semiconductor stack 10 is produced according to the secondembodiment in such a way that the layer of interest 12 based on GeSn hasa direct electronic band structure, and then the semiconductor structure40 is produced by the process according to the fourth embodiment(hydrophilic bonding). Then the doped zones 43, 45 are produced byimplantation of impurities, for example phosphorus and boron. Theelectrical contacts 46A, 46B are then produced. An encapsulation layer42 is deposited and then smoothed by a mechanical-chemical polishingtechnique of the CMP (chemical mechanical polishing) type and thenetched locally so as to make the electrical contacts accessible.

FIG. 7B illustrates a variant of the light-emitting diode illustrated inFIG. 7A, which differs from it essentially in that a p-i-n junctionextends approximately parallel to the plane of the supporting layer 1.

The central portion 21 is structured in its thickness, so as to have alower part 43 based on GeSn doped according to a first type ofconductivity, here of p-type, based on the nucleation layer 11 (notshown). This p-doped part 43 is connected to the tensioning arms 22 andhas a mean thickness roughly identical to that of the arms. An upperpart 45 based on GeSn doped according to a second type of conductivity,here of the n-type, rests on the p-doped lower part. An intrinsic part44 based on GeSn is located between the n-doped upper part 45 and thep-doped lower part 43, and in this case has dimensions in the (X,Y)plane roughly identical to those of the upper part. The intrinsic part44 corresponds advantageously to the layer of interest 12 with a directelectronic band structure (not shown). Thus, the p-doped and n-dopedparts 43, 45 and the intrinsic part 44 together form a p-i-n junctionthat extends in a plane approximately parallel to the (X,Y) plane. Twostuds 46A, 46B of an electrically conducting material, formingelectrical contacts, are arranged on the n-doped upper part and on afree zone of the p-doped lower part.

FIGS. 8A and 8B show a schematic sectional view of the examples of anoptoelectronic device for emitting coherent light. More precisely, theoptoelectronic device is in this case an optically or electricallypumped laser source.

In FIG. 8A, the laser source in this case comprises a semiconductorstructure 40 obtained by the production process according to the fourthembodiment, i.e. involving hydrophilic molecular bonding. The lasersource in this case comprises a semiconductor structure 40 formed from acentral portion 21 of the stack 10 in tension with a layer of interest12 based on GeSn (not shown) with an advantageously direct electronicband structure, and made integral with the supporting layer 1 byhydrophilic molecular bonding.

Here, the layer of interest 12 of the semiconductor structure 40 isintrinsic or even doped, for example with phosphorus to populate theindirect valley of the conduction band, and an optical cavity isproduced, inside which the central portion 21 is located, which in thiscase forms a gain medium able to emit light. For this, and for purposesof illustration, two Bragg mirrors 47A, 47B are arranged on the upperface of the tensioning arms 22, preferably in a zone where the strain ofthe tensioning arms 22 is approximately equal to the residual value.

FIG. 8B illustrates a variant of the light-emitting diode illustrated inFIG. 8A, which differs from it essentially in that a p-i-n junctionextends approximately parallel to the plane of the supporting layer 1 inthe central portion 21. The central portion 21 in this case comprises astack of a first lower part 43 based on GeSn, located near thesupporting layer 1 (and separated from the latter by the nucleationlayer 11), doped according to a first type of conductivity, for exampleof p-type, covered with an intrinsic intermediate part 44 correspondingto the layer of interest 12 (not shown), itself covered with an upperpart 45 based on GeSn doped according to a second type of conductivityopposite to the first type, for example n-type. An optical cavity,similar to that described with reference to FIG. 8A, is made at thelevel of the upper face of the tensioning arms 22. Moreover, twoelectrical contacts (not shown) are provided to be in contact, one withthe n-doped upper part, and the other with the p-doped lower part.

FIGS. 9A to 9F show schematic sectional views of different steps of anexample of a process for producing a laser source in which the opticalcavity is made at the level of the supporting layer 1.

In this example, a layer of a semiconductor material 8, here ofgermanium, is produced, for example by RP-CVD (reduced-pressure chemicalvapor deposition) epitaxy, on a silicon substrate 7 (FIG. 9A). Then thegermanium layer 8 is covered with a layer of oxide 9 and then H+ ionsare implanted in the germanium layer 8 (dotted line in FIG. 9B). Then asilicon layer 1 is produced, intended to form the supporting layer 1,here in the form of a SOI substrate, in which two Bragg mirrors 47A, 47B(or equivalent optical elements) are produced at the level of itssurface, intended to form an optical cavity. The surface of thesupporting layer 1 is then covered with a layer of oxide. The SOIsubstrate is attached to the surface of the layer of oxide 9 (FIG. 9C).The germanium layer 8 is fractured at the level of the ion implantationzone and we thus obtain an assembly of a layer of germanium forming thenucleation layer of the stack 10, bound to a silicon supporting layer 1via a sacrificial layer 2 of silicon oxide. The two Bragg mirrors 47A,47B are buried in the supporting layer 1 at the level of the interfacewith the sacrificial layer 2 (FIG. 90).

Then the semiconductor stack 10 is produced according to the first orsecond embodiment, comprising a layer of interest 12 based on GeSn (notshown) preferably with direct electronic band structures. Then asemiconductor structure 40 is obtained on the basis of the processaccording to the second embodiment. The Bragg mirrors are thus arrangedopposite the tensioning arms 22, or even opposite the central portion21, and surround the central portion 21 so as to form an optical cavity(FIG. 9E). Then an encapsulation layer 42 is deposited, for example ofsilicon oxide, which covers the semiconductor structure 40. Finally ap-i-n junction 43, 44, 45 is produced through the middle layer 21 andtherefore the layer of interest 12 (not shown), and then electricalcontacts 46A, 46B are made (FIG. 9F). Moreover, the supporting layer 1may have been structured beforehand so as to form the core of awaveguide surrounded by a sheath formed from silicon oxide; the coreextends roughly opposite the central portion 21.

Particular embodiments have just been described. Different variants andmodifications will be apparent to a person skilled in the art.

Thus, the optoelectronic devices described above are only described byway of illustration. Other optoelectronic devices may be produced, forexample optically or electrically pumped laser sources, with or withoutp-n, p-i-n junctions, or light-emitting diodes or photodetectors.

1. A process for producing a tensionally strained layer based ongermanium-tin, the process comprising: a) producing a semiconductorstack resting on a supporting layer via a sacrificial layer, saidsemiconductor stack comprising a nucleation layer and a layer based ongermanium-tin grown epitaxially from the nucleation layer, saidsemiconductor stack having a non-zero initial value of tensile stress;b) structuring said semiconductor stack so as to form: a structured partand a peripheral part, the structured part comprising a central portionjoined to the peripheral part by at least two lateral portions oppositeone another with respect to the central portion, and the lateralportions having an average width greater than an average width of thecentral portion; and c) suspending the structured part by etching thesacrificial layer located beneath the structured part, the suspendedcentral portion then having a final value of tensile stress greater thanthe initial value.
 2. The process as claimed in claim 1, furthercomprising: prior to the producing a), estimating a value of atomicproportion of tin and a first minimum value of tensile stress for whichthe layer based on germanium-tin has a direct electronic band structure;and determining the semiconductor stack comprising the nucleation layerand said estimated layer based on germanium-tin, and having a secondminimum value of tensile stress; producing said semiconductor stack insuch a way that it has said non-zero initial value of tensile stress andso that the layer based on germanium-tin has an initial value lower thansaid first minimum value; determining the structuring in such a waythat, after said suspending c), the central portion of the structuredpart has a final value of tensile stress greater than or equal to saidsecond minimum value, said layer based on germanium-tin then having afinal value of tensile stress greater than or equal to said firstminimum value and then having a direct electronic band structure.
 3. Theprocess as claimed in claim 1, wherein the nucleation layer is made of asemiconductor compound having a natural lattice parameter lower thanthat of the material of the layer based on germanium-tin.
 4. The processas claimed in claim 1, wherein the semiconductor stack comprises atleast one layer located between the layer based on germanium-tin and thenucleation layer and made of a semiconductor compound having a naturallattice parameter less than or equal to that of the material of thelayer based on germanium-tin.
 5. The process as claimed in claim 1,wherein the semiconductor stack has a thickness less than a criticalthickness.
 6. The process as claimed in claim 1, wherein each layer ofthe semiconductor stack has a thickness less than a critical thickness.7. The process as claimed in claim 1, wherein the semiconductor stackcomprises an upper and a lower layers based on germanium-tin, which aredoped according to different types of conductivity and located on eitherside of the layer based on germanium-tin, which is not dopedintentionally.
 8. The process as claimed in claim 7, wherein, betweenthe upper and lower doped layers on the one hand and the layer based ongermanium-tin on the other hand, there is at least one barrier layerbased on germanium, or based on germanium-tin whose atomic proportion oftin is lower than an atomic proportion of tin in the layer based ongermanium-tin.
 9. The process as claimed in claim 1, wherein an atomicproportion of tin in the layer based on germanium-tin is below 10%. 10.The process as claimed in claim 1, further comprising contacting thestructured part with a freed surface of the supporting layer, so as tomake the structured part integral with the supporting layer by molecularbonding.
 11. The process as claimed in claim 10, further comprising:determining a minimum value of molecular bonding energy of thestructured part on the supporting layer, as well as a minimum value ofbonded surface area of the lateral portions, these minimum values beingsuch that said energy of molecular bonding is greater than an elasticenergy of the structured part; consolidation annealing at an annealingtemperature such that the energy of molecular bonding has a valuegreater than or equal to said previously determined minimum value; andthen etching a distal part of the lateral portions with respect to thecentral portion, in such a way that the bonded surface of the lateralportions has a value greater than or equal to said previously determinedminimum value.
 12. The process as claimed in claim 11, wherein saidsuspending and said contacting are carried out by etching thesacrificial layer with HF in a vapor phase optionally followed bydeposition and then evaporation of a liquid between the suspendedstructured part and the supporting layer, and in said consolidationannealing, the annealing temperature is greater than or equal to 200° C.13. The process as claimed in claim 11, wherein, in said suspending,oxidation or nitriding of a freed surface of the supporting layer aswell as of a lower surface of the structured part oriented towards thefree surface is performed, and in said consolidation annealing, theannealing temperature is greater than or equal to 100° C.
 14. Theprocess as claimed in claim 13, wherein, at the end of said suspending,dielectric layers resulting from the oxidation or nitriding are formedat the level of the structured part and of the supporting layer.
 15. Aprocess for producing a microelectronic or an optoelectronic devicecomprising a tensionally strained layer based on germanium-tin obtainedby the process as claimed in claim 1, the process comprising: producinga p-n junction in the tensionally strained layer, or a p-i-n junction atthe level of the tensionally strained layer, the latter then not beingdoped intentionally.